Easily done, I bought a pack of three different colours for £9 with free postage from eBay.

2022-09-03

Laser arrived. 650 nm red laser very bright, 405 nm blue laser rather dim, 532 nm green laser dead. Not even a faint glow.

This was with NiMH cells, 1.2 volts each, 2.4V total. Maybe the non-red lasers need alkaline cells of 1.5 volts each, 3V total. This had the same results.

Swapping the battery barrel was not possible, as the green laser has a different thread. But it was possible to hold a different barrel in place long enough to verify the laser actually worked.

Poking a screwdriver down the middle of the barrel, I managed to measure the resistance between it and the barrel as open circuit. The barrel is metal-coated plastic, maybe the coating was missing at the end. Maybe a bit of scrunched-up aluminium foil would help? Pressing this in with the batteries pushed the end of the battery barrel out, allowing a good look at the coating. My guess was right, very poor coating. Reassembled, the green laser was much brighter than the red. 

This is a bit worrying. You might think it is cool to have a brighter laser than you paid for, but you REALLY DO NOT WANT anything more powerful than five milliwatts. One milliwatt is fairly safe, five is borderline and anything above is well dodgy. One 9-year old boy was blinded playing with a laser his mum bought from a toy shop:

https://metro.co.uk/2018/09/05/boy-blinded-himself-after-shining-laser-in-his-eye-after-his-mum-told-him-not-to-7915033/

It is clear that there is little or no control on the sales of lasers. The technology that makes them cheap enough to be used in consumer electronics also makes them cheap enough to be sold as toys. There are lasers for sale on eBay in the 500 or even 2000 mW range. They really ought to be made highly illegal to own without a damn good reason and strict safety measures in place. 

DO NOT dick around with lasers. Stupid people regularly get arrested for shining lasers at people or aircraft. They are not toys for playing with cats either.

https://www.gov.uk/government/publications/laser-radiation-safety-advice/laser-radiation-safety-advice

Another reason for the relative brightness of the green laser is that the human eye is more sensitive to green:

This graph shows green appears about ten times brighter than red. The 'blue' laser of 405 nm is in the very insensitive zone, and looks more like purple.